Conformational dependence of substituent effects in the solvolyses of the 1,1-diphenyl-2,2,2-trifluoroethyl system

The substituent effects on the solvolysis of 1-X-phenyl-1-Y-phenyl-2,2,2-trifluoroethyl tosylates were analyzed on the basis of the Yukawa-Tsuno equation. For the solvolysis of the symmetrically disubstituted X = Y subseries, an excellent linear correlation was obtained. However, in the solvolyses of series of varying X with a fixed-Y subset, the substituent effects were found to give significantly concave Y-T correlations, The rho value for the Y-T correlations changes significantly with the fixed Y substituent. There is a qualitative trend of a linear decrease in the rho value as the fixed-Y substituent of the respective subset becomes more electron donating. The non-linearity of substituent effects was attributed to a substituent-induced conformational change of the transition state, which could be simulated by calculation of the preferred conformation of the intermediate carbocation. A molecular orbital optimization method was applied to determine the preferred conformations of the alpha,alpha-diarylcarbenium ions derived from the title systems. The symmetrical carbenium ions X = Y have a preferred propeller shape conformation with twist angles differing by 14degrees (E conformation), whereas in the unsymmetrical systems X not equal Y the two aryl rings are much more (>30degrees) differently twisted in the preferred conformation (PT conformation). The linear correlation found for the symmetrical subseries is essentially an outcome of the E conformation of the transition state. In unsymmetrical cases, the substituent effect correlation in any Y subset should reflect the conformational arrangements, E-X, P-X and T-X of the variable X-aryl group, depending on relative (σ) over bar values of X and Y. The substituent effects in the Y-subsets were successfully treated by three different Y-independent correlations for the preferred conformational arrangements: E correlation for substituents when ((σ) over bar (X) - (σ) over bar (Y)) congruent to 0, P-X correlation when ((σ) over bar (X) - (σ) over bar (Y)) much less than 0 and T-X correlation for the ((σ) over bar (X) - (σ) over bar (Y)) much greater than 0 class, respectively. Copyright (C) 2002 John Wiley Sons, Ltd.